Polyvinyl alcohol (hereafter, also referred to as PVA) has been widely used as a packaging material because it is excellent in transparency, oil resistance, chemical resistance, and gas barrier properties against oxygen or the like. PVA has been often used as a packaging material for food, pharmaceuticals, industrial chemicals, agrochemicals, and the like whose properties are greatly affected by deterioration due to oxidation.
In addition, PVA has been also used as a film or sheet for temporary protection of products and parts because of its high water solubility. PVA used as a film for temporary protection, such as surface protection during metalworking, protection during vulcanization of rubber parts, and surface protection of resin molded articles, can be easily removed by water, warm water, or hot water. This enables omission of a step of peeling and discarding the protective films.
However, the water solubility of PVA is problematically lowered when PVA is subjected to a heat history during the surface protection. For example, in metalworking, the protective film may be subjected to a heat history in the step of heat pressing or the like. In such a case, PVA may not be sufficiently removed due to the lowered water solubility.
Upon use of a PVA film for packaging chemicals or the like, for example, the film wraps chemicals first and is formed into a bag, during which the film is unwound and fed continuously.
However, PVA disadvantageously has high hygroscopicity to have insufficient elasticity when formed into a film, namely, a PVA film has low stiffness. A PVA film therefore has poor surface slipperiness, resulting in poor unwinding properties of the film. As a result, there may be failures such as a shift of pitches or wrinkles of the film upon heat-sealing in a bag making machine. In other words, such a PVA film disadvantageously has low process passing properties.
To overcome the situation, Patent Literature 1 discloses a technique of adding 20 to 40 parts by weight of a plasticizer and 10 to 35 parts by weight of starch to 100 parts by weight of polyvinyl alcohol to inhibit reduction of water solubility and improve heat sealing properties and process passing properties.
However, even with the technique disclosed in Patent Literature 1, the stiffness of the film is still insufficient, resulting in poor unwinding properties. Moreover, simply adding starch to a water-soluble film may fail to give an effect of improving unwinding properties, failing to improve the process passing properties. In addition, in the technique disclosed in Patent Literature 1, a large amount of starch needs to be added. However, in a case where a large amount of starch is applied to the surface of a water-soluble film, a lot of starch is deposited on a surface of a conveying roll upon packaging of chemicals such as agrochemicals or detergents, which requires an additional step of cleaning the conveying roll, resulting in lower productivity.
Patent Literature 1 also discloses a technique of embossing the surface of a water-soluble film for the purpose of improving anti-blocking properties and process passing properties of the water-soluble film.
The embossing treatment however lowers transparency of the water-soluble film to reduce the visibility for determining if chemicals such as agrochemicals or detergents are contained therein.
Patent Literature 2 discloses a technique of improving the water solubility after a heat history by adding a hydrazine compound and a polybasic acid or an alkali metal salt thereof to a polyvinyl alcohol resin.
However, according to the technique of Patent Literature 2, the film disadvantageously exhibits poor processability upon packaging contents due to its low flexibility and has insufficient water solubility, especially resulting in a long dissolution time thereof.
A water-soluble packaging film is often formed into a bag first and then sealed with heat or the like. In such a case, the sealed portion tends to have lower durability, which leads to a defect in the sealed portion by an influence of the environment or the like after a lapse of a long period of time from inclusion of the contents, so that the film cannot wrap the contents stably over a long period of time.
To overcome the situation, Patent Literature 3 discloses a water-soluble film prepared by adding a plasticizer and sulfite to a polyvinyl alcohol resin. Such a film suffers less coloring upon film formation or less coloring with time due to contact with chemicals.
However, addition of sulfite to a water-soluble film cannot shorten the dissolution time. In addition, sulfite may react with a weak acid to generate sulfur dioxide gas, which causes offensive odors from the water-soluble film.